Hydraulic presses are used when workpieces are to be formed or reformed. Hydraulic presses are also used for cutting operations. The required force of the hydraulic press depends on the workpiece. In the ceramic industry, presses having a pressing force of 20,000 kN or more are used. In this case, with a view to efficient manufacture, the cycle time for a pressing operation should be as short as possible. Cycle sequences of 20 strokes per minute are a guideline. The pressing force and the cycle time determine the energy to be expended, that is to say, in hydraulic presses, the power of pumps and of electric motors driving these pumps. In hydraulic presses according to the prior art, accumulators are also used, such as pressure medium accumulators or flywheels.
DE A1-43 20 213 discloses a hydraulic press that includes a feed circuit of the hydraulic pressing cylinder. A pressure medium accumulator is charged during the return stroke of the press and is utilized for the drive during the feed of the pressing die. Energy can thus be saved in the main drive.
JP-A-63 256 300 discloses a press which is operated with a multistage pressure converter. After a first pressing operation at low pressure, the hydraulic oil is discharged into the tank. A second pressing operation then takes place at high pressure. Energy recovery is consequently not possible in this case.
A hydraulic drive system for a press in the relevant field is disclosed in U.S. Pat. No. A-5,852,933 and DE A1-44 36 666. It contains a low-pressure and a high-pressure circuit. In this, there are three hydrostatic machines, two of which are coupled mechanically. In order to make satisfactory operation possible, these machines must be adjustable in terms of their absorption volume or delivery volume. This entails considerable costs. The system described here can be employed only when the press has differential cylinders or synchronous cylinders.
DE-A1-43 08 344 discloses the principle of secondary regulation for regulating the drive of a hydraulic press. The various movements of the press ram are combined with one another in such a way that the pressure network operates in a closed circuit, the maximum system pressure being determined by the pressure medium accumulator.
DE-A1-43 08 344 discloses that the hydraulic oil is definitely compressible also plays a part in the regulation of a hydraulic press. This has an effect in a press cycle during both compression and decompression and constitutes a source of losses.
The prior art has continued largely to ignore the fact that the mechanical parts of the press also absorb energy due to the elastic deformation of their components. This energy has to be expended during the closing operation of the press. This energy is not recovered during the opening operation.